As far as is known, this is the first study to concentrate on the microstructural deformation of clay is undrained shear. The material was naturally dispersed; and it had been expected that during pre-peak deformation the platy clay particles would slip and turn to form large domains (i.e. groups of sub-parallel particles) inclined at 45o or more, although other hypotheses were also under consideration. Both optical microscopy and electron microscopy showed that the preferred orientation, which was originally horizontal, definitely remained horizontal whilst the shear strain increased, until the failure plane cut through the orientated structure. It was expected that when strongly anisotropic clay samples are deformed, the shear strain will disrupt the structure, so that the strength of anisotropy decreases. Conversely, when isotropic clay samples are deformed, it was expected that slippage of the particles will cause the strength of anisotropy to increase. The present samples started at a moderate strength of anisotropy, and the subsequent changes were small. This observation supports the above hypothesis and suggests that the examples were on the borderline between the two types of behaviour. There were, however, some changes of the microstructure. The strength of anisotropy at first appeared to increase and later to decrease as strain increased. The deviations of the orientations of the individual structural elements appeared to decrease at first and later to increase, this being the complementary trend. In addition, post-peak, the between-samples standard deviations increased with strain, suggesting that different samples were deforming in different ways. These results suggest that in normally consolidated undrained shear there may be three mechanisms acting within these samples, i.e. they were bedding down at first, and later being disrupted by the shear strains; but the failure plane cut through all pre-existing structure non-conformably. Further, there may have been an element of instability in the microstructural response to strain. The results available for the outer series of tests led to broadly similar conclusions.